Tiwanaku was in a marshy area subject to periodical inundation, and seems to have had canals for transport. It seems to me like if something falls in a canal, and past a point it's no longer maintained, it can get easily buried in whatever the water and the wind blow into it.
The agricultural land was also on raised platforms, to control the flooding. I.e., there was a lot of soil which had been dug up and placed higher than normal. Again, it seems to me like no miracle that a lot of stuff would be blown or washed off the high places, and end up in the low places, i.e., the canals. Gravity tends to do that.
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